Excessive daytime sleepiness (EDS), is the most common complaint of subjects suffering from obstructive sleep apnea hypopnea syndrome (OSAHS). The broad long term objective of this research is to examine the complex relationship between the physiological stress, sleep disordered breathing (SDB), its associated sleep fragmentation, and its outcome (daytime sleepiness). Because correlations between conventional measures of fragmentation and measures of sleepiness have been poor, SDB will be assessed over multiple nights using the best available technology for identifying obstructive respiratory events and a novel measure, the obstructive index (Ol) will be obtained from the events detected. These will be related to EDS, quantified using multiple measures including the psychomotor vigilance task, the Multiple Sleep Latency Test and subjective scales of sleepiness. These relationships will be evaluated in subjects with suspected OSAHS following monitored treatment with nasal Continuous Positive airway Pressure (CPAP) and in normal volunteers. The specific aims of the study are Specific Aim 1: Evaluate the correlation of our integrated Obstruction Index (Ol) to sleepiness in untreated subjects with a range of severity of obstructive sleep disordered breathing. In a subanalysis, the relative contributions to EDS of events with different characteristics (eg O2 desaturation, EEG and autonomic arousal, changes in upper airway resistance) will be evaluated to further refine the Ol if it can be shown they contribute additional predictive power of the index for EDS. Specific Aim 2: Test the utility of the Obstruction Index to predict changes in sleepiness associated with changes in SDB following treatment with CPAP. Specific Aim 3: Experimentally introduce SDB through suboptimal CPAP therapy to demonstrate a "dose-response" relationship of SDB (using the Ol) to EDS. Specific Aim 4: Use the Obstruction Index to predict sleepiness following an acute period of SDB induced by a brief withdrawal of CPAP. Defining these relationships not only has implications for our understanding of this disease, its diagnosis and treatment, but also contributes to a more general understanding of the relationship between sleep and sleepiness. Public Health Relevance: Obstructive sleep apnea/hypopnea syndrome affects 2-4% of the adult U.S. population. Excessive daytime sleepiness is the primary problem in these patients. The present study contributes to understanding these symptoms of sleepiness in patients with sleep apnea. Validating a measure of sleep disordered breathing that relates to sleepiness will help improve the diagnosis and treatment of obstructive sleep apnea.